There are 2 ways to grow an Avocado tree.

Grafted or Non Grafted

Grafted Avocado trees produce in 2-3 years, this is where all your popular types come from, Hass, Bacon, Pinkerton, Lamb.  The Hass Avocado (1935-2003) was one tree that produced a favorable fruit, the mother tree of all Hass trees, but it was not her seeds that produces all the other Hass Avocado trees, but her cut branchs where grafted to less desirable trees.  This created clones of the orginal tree,(which died in 2003 of root-rot)

The only way to get a Hass Avocado tree is through grafting..........

Non-Grafted / Experimental trees these can take 10-13 years to produce fruit if at all, just like in life when 2 people have a child, you never know what your going to get, you can guess that it will be like the parents but you and with trees you might not know who the other parent is..

The Avocado seed is the same way the Genes have been mixed up and the dice have been rolled, and what you might get nobody knows.  That makes it exciting though.

If your Non-Grafted tree does not produce fruit it can all ways be grafted to, and with the allready, established root system (root-stock) the tree can produce within a year if your lucky but definitly in 2-3 years. 

I've seen to many people selling Hass, Bacon, Pinkerton  etc,,, on eBay that are not grafted trees, I sorry but this in not possible